opal-rspec does not currently have a means to change the spec directory location, which you really need to do if you are going to have mixed opal and regular (server) ruby code testing.
The following assumes normal specs will go into the the spec
directory, and opal specs will go into spec-opal
directory.
To get this to work, you have to change the Opal::Server
path to end with spec-opal
instead of spec
which is what opal-rspec normally does. Then you have to change the Opal::Server main file (the sprockets_runner.rb.erb) file to be a copy which pulls in your specs.
In your rake file do this:
require 'opal/rspec/rake_task'